Hard rubber is normally a rubber that has been compounded with various ingredients to increase its hardness. A measure of a harder rubber is typically demonstrated by a physical property such as a shore "A" ASTM hardness test. (ASTM No. D2240) Generally, a shore "A" hardness value in the range of about 80 to about 100 indicates that such a cured rubber would be classified as being a relatively hard rubber.
Generally the term "stiff rubber" can be exemplified by a physical property such as a three point bend test value (ASTM No. D-4475-85) and, generally, three point bend test values in the range of about 20 to about 30 denotes a stiff rubber (with a modified ASTM D-4475-85 as to sample shape and size, instrument crosshead speed and data points as hereinafter referenced in the example of this specification).
A hard rubber, although often more stiff than a rubber of less hardness, may not necessarily be, and more typically would not be, considered a relatively stiff rubber under such hardness parameters. Indeed, it would not usually be expected that a typical rubber of shore "A" hardness in the range of about 80 to 100 would have the prescribed three point bend test value of at least 20.
Stiff rubber compositions are often desirable for various purposes such as, for example, industrial rubber products of various types--also various components of an automobile tire, particularly those positioned in the bead and associated sidewall region.
Relatively hard rubber which may have a degree of stiffness can conventionally be prepared, for example, by compounding or mixing a relatively large amount of reinforcing materials with the rubber prior to its vulcanization such as, for example, carbon black, silica usually with a coupling agent, and short discontinuous fibers.
However, stiffer rubbers are sometimes desired which, in their unvulcanized state, can still be processed with conventional rubber processing equipment.
Conventionally, such stiffer rubber compositions are often prepared by adding various hardening resins and other such materials, as well as various reinforcing materials, to rubber mixtures as a part of their compounding recipe following which the composition is shaped and cured to form a stiff rubber material which can be a component of a manufactured article.
However, it is still desired to prepare additional suitable rubber compounds having a high degree of stiffness but which substantially maintain other desirable physical properties such as processability of the compounded rubber itself during its unvulcanized mixing stage and, its extrusion stage if utilized, and, also, the property of adhesion to other tire rubber components and to itself. For example, a stiff apex component of a tire should adhere to its associated sidewall and chafer components.